A continuous flow steam generator includes a combustion chamber, having substantially rectangular cross-section and a lower and upper combustion chamber region, and has a horizontal gas pass connected downstream of the combustion chamber on the flue-gas side. Gas-tight and gas-permeable peripheral walls of the generator are completely or partly made of steam generator pipes welded together and through which a flow medium can flow, and collectors are arranged and connected to the steam generator pipes such that groups of steam generator pipes connected in parallel form heating surface segments of the peripheral walls. First passage collectors are arranged and connected such that the flow medium from first heating surface segments of two parallel first peripheral walls of the lower combustion chamber region are mixed with the fluid medium from second heating surface segments of second peripheral walls, standing perpendicular to the first peripheral walls, of the upper combustion chamber region.